The use of metal devices permanently implantable in bone tissue is widespread in various branches of medicine. For example, the dental implant surgery provides the use of screws, generally made of titanium, in the lower jaw or maxilla bones to artificially replace lost or no longer functional bone roots. In orthopedics, several devices for fracture fixation, reduction of vertebral mobility, vertebral column surgery, are commonly implanted in bone tissue.
In these applications, the implanted device is firmly locked in the implant site due to the growth, until direct contact with the device, of newly formed bone tissue. This phenomenon, which is known as the osteo-integration, has been widely studied and described in the technical-scientific literature of the sector, particularly as refers to the bone implant surgery by means of titanium devices. Contrarily to other processes of implantation of foreign material in tissues, entailing the encapsulation in a fibrous material, i.e fiber integration, the growth of bone tissue directly in contact with the device offers a firm anchorage, which makes the device suitable to withstand loads and perform structural tasks.
While the disciplines based on osteo-integration are recently having great success and ever-increasing applications, several problems still remain to be solved. Particularly, it is important to accelerate the osteo-integration process as much as possible, thereby reducing the time between the insertion of the implant and the actual load thereon. For example, in dentistry the implant is generally not “loaded”, thereby the patient cannot perform his masticatory function by means of that implant, for a period of time ranging from 1 to 4 months after the intervention, in order to allow bone tissue healing and induce osteo-integration. Furthermore, while bones normally heal well with healthy and young people, it is often very slow with old and osteoporotic people, i.e. those more likely to require these interventions and being a significant portion of patients requiring implant operations for trauma or fixation of spine mobility.
As it is generally known that the surface properties of the implant devices play a basic role in the tissue response to the implant, a great number of researches has been carried out to improve the osteo-integration process by modifying the surface of the implantable devices. A detailed picture of these researches is set forth in “The bone-biomaterial interface” by Puleo and Nancy, Biomaterials 1999; 20:2311-2321, or the textbook Bone Engineering, Davies, published by EM SQUARED, Toronto, 2000. From studying these books and evaluating the devices being marketed, it is understood that the improvement of the surface properties is often pursued by means of surface roughening, for example by means of sandblasting, plasma-spray deposition or treatments with acids. The deposition of layers of ceramic materials with high bone affinity, such as the hydroxyapatite or the so-called bioglass, has also been studied and applied.
In addition to these methods, great interest has arisen in introducing on the surface of the implant devices biological molecules capable of promoting bone growth. Among the most studied molecules, there has been reported that the collagen, when immobilized to the surface of an implant titanium screw, can increase the osteo-integration speed. Particular peptides, i.e. small molecular fragments composing proteinic molecules, which are capable of interacting particularly with bone cells, have also proved effective when tested in vivo. To the purpose, the above-mentioned article by Puleo and Nancy studies the several molecules used for carrying out the biochemical modification of implant surfaces.
Though the biochemical modification of implant surfaces is a sector of great scientific and speculative interest, its practical application still has considerable problems. The collagen, for example, has problems of contamination as it originates from dubious animal sources (particularly, bovine collagen) or rejection due to possible incompatibility reactions among different species. The above-mentioned peptides are rather costly and poorly stable from a chemical point of view, such that resorting to the typical procedures of the sector, for example the sterilization, in the treatment of implant surfaces is hardly feasible.
The hyaluronic acid is a glycosaminoglycan diffused in all tissues of living beings, without any variation among species. It has very interesting biochemical and hydration characteristics and for this reason it is widely studied and used in various specialties within the biomedical field. An exhaustive overview of the application of the latter is set forth, for example, in some works containing the proceedings from the main conferences on hyaluronic acid: “The Biology of Hyaluronan”, D. Evered and J. Whelan, Eds. Wiley, Chichester, 1989, “The Chemistry, Biology and Medical Applications of Hyaluronan and its Derivatives”, T. C Laurent, Ed., Portland Press Ltd, London, 1998, “Redefining Hyaluronan”, G. Abatangelo and P. H. Weigel, (Eds.), Elsevier, Amsterdam, 2000, “Hyaluronan”, J. F. Kennedy, G. O. Phillips, P. A. Williams, V. Hascall, Eds., Woddhead Publishing Limited, 2002.
The hyaluronic acid, as a molecule in the homogeneous phase, plays an active role in the bone formation process, such as described, for example, by Bernard et al. in the above-mentioned work “Redefining Hyaluronan”, G. Abatangelo and P. H. Weigel, (Eds.), Elsevier, Amsterdam, 2000, p. 215.
For this reason, hyaluronic acid-based gels imbibed of bone morphogenetic proteins or growth factors, have been successfully used in bone stimulation tests. Furthermore, it has been demonstrated that hyaluronic acid solutions, optionally coupled with the dexamethasone drug having osteogenic properties, exert a positive effect on the specialization in bone cells of marrow stromal cells, such as described by Zou et al., Biomaterials, 2004; 5375-5385, 25.
However, the interesting osteogenic potentiality of the hyaluronic acid, either as a gel or in solution, or the hyaluronic acid present in tissues cannot be immediately used in the bone tissue implantation devices as described above. In fact, the hyaluronic acid is very soluble in aqueous solutions and its time of permanence in situ is very short. Chemical techniques favouring the permanence of the hyaluronic acid in the implant site, such as cross-linking, chemical modification or surface immobilization, can alter the structure and molecular conformation of the hyaluronic acid and negatively affect the receptor-ligand specific interactions, thereby compromising the bioactive behaviour of the molecule. In fact, the bioactive properties of the hyaluronic acid derive from its capacity of interacting with specific receptors located on the cell wall, such as CD44 or RHAMM. Such as described by J. Lesley et al., J Biol. Chem. 2000 Sep. 1; 275(35):26967-75, this type of interaction is highly co-operative and, in order to be effective, requires the simultaneous interaction of many repeats of hyaluronic acid with a single receptor. The co-operative nature of the interaction implies the typical mobility of molecules in solution, therefore the immobilization of hyaluronic acid on material surfaces, such as described by Morra and Cassinelli, Journal of Biomaterials Science, Polymer Edition, 1999; 10(10):1107-24, leads to surfaces that do not allow any cell adhesion due to the inability of establishing specific interactions being sufficiently strong. The reduced adhesion of cells or biomolecules to surfaces with immobilized hyaluronic acid is substantiated in various scientific literature articles and is used, as set forth by Witt at al., “Hyaluronan”, J. F. Kennedy, G. O. Phillips, P. A. Williams, V. Hascall, Eds., Woddhead Publishing Limited, 2002, volume 2, p. 27, to reduce adhesion phenomena subsequent to surgical operations. The hyaluronic acid immobilization on metal substrates and devices has been reported by Pitt et al., in the article: “Attachment of hyaluronan to metallic surfaces”, issued in Journal of Biomedical Materials Research, vol. 68, p. 95, 2004. In accordance with the general knowledge, such as described above, in the cited article the surfaces with immobilized hyaluronic acid thereon are designated as being “biopassive” or with poor cell adhesion. The Authors of the article point out how the poor biological adhesion imparted by the immobilized hyaluronic acid layer can be used to prevent non-specific adhesion; and how, in order to obtain a specific bio-adhesion effect, it is necessary to bind adhesion peptides to this non-adhesive matrix.
Essentially, it is generally acknowledged that hyaluronic acid layers immobilized on solid surfaces have characteristics of resistance to biological adhesion, which is contrary to what one would desire to obtain by means of the bioactive action of hyaluronic acid immobilized on implantation devices, wherein the specific cell adhesion of the implant to the bone tissue is crucial for osteo-integration.
It is further acknowledged that the bone neoformation process requires a mineralization step being promoted by calcium ions binding to the surface. As described by Bernard et al. in the above-mentioned work, the hyaluronic acid, in nature, has an active effect in this step, thereby significantly contributing to the calcification process. The hyaluronic acid carboxylate groups can, in fact, chelate or complex calcium ions by exerting a positive action on the mineralization process. However, the immobilization of hyaluronic acid on the surface of implant devices normally implies binding the hyaluronic acid carboxylic groups with aminic or hydroxyl functionalities present on the substrate, with the consequent loss, in the bound hyaluronic acid, of carboxylic groups being available for chelation with calcium ions. Accordingly, the immobilization of the hyaluronic acid on the surface of these devices by the known methods does not lead to any improvement in the osteo-integration process.
On the other hand, the present applicant has surprisingly found that hyaluronic acid immobilized on implant screws, in accordance to what is set forth in the annexed claims, has an active effect on the osteo-integration process in vivo, without any further peptide immobilization being required, and that the properties of those devices being implantable by contact to bone tissue having a layer of immobilized hyaluronic acid according to the invention are definitely improved compared to conventional devices.